Structural and metamorphic implications of the final emplacement of the Lyngen Nappe

Abstract

The Lyngen Nappe, an allochthonous unit of the Northern Norwegian Caledonides, consists of Greenschist-facies rocks overlying Amphibolite- to Granulite-facies rocks in the Nordmannvik Nappe. The lower-grade metamorphism seen in the Lyngen rocks juxtaposes higher-grade rocks both above and below. The nature and origin of this metamorphic break is investigated, specifically in the sheared rock units comprising the contact between the Lyngen and Nordmannvik nappes, to determine the kinematic and metamorphic conditions present during the final emplacement of the Lyngen Magmatic Complex. These mafic rocks primarily consist of the Lyngen Gabbro, part of an ophiolite sequence formed in the former Iapetus Ocean, and now overly the metasedimentary paragneisses of the Nordmannvik Nappe that formed the Baltican basement. These nappes were emplaced and deformed during the Scandian orogenic phase of the Caledonian Orogeny. Structures observed in the field strongly indicate a top-to-the-West shear sense at this contact, while petrologic modelling of the rocks in this shear zone provide evidence for prograde development. Estimated metamorphic conditions of the Nordmannvik Gneiss (625-675°C and 8.5-10 kB) represent minimum values during Scandian shearing for temperature and pressure when compared to previous estimates (680-710°C and 9.4-10.1 kB [Faber, pers. comm., 2017]). Above the Nordmannvik rocks, a trend of decreasing temperatures and pressures is observed, with conditions of 580-600°C and 10-10.5 kB, followed in the higher rocks by estimates of 540-550°C and 7.5-8.25 kB. Structural and mineralogical similarities between the Garnet Mica gneisses of the Nordmannvik Nappe and the overlying phyllites and schists, however, hint at a common origin through retrograde metamorphism. An extensional detachment for the Lyngen Nappe is proposed, within a few possible tectonic models.